This invention relates in general to semiconductor lasers and in particular to surface emitting semiconductor lasers.
Surface emitting semiconductor lasers are advantageous over other types of semiconductor lasers for a number of reasons. In the surface emitting semiconductor laser diode device, radiation is emitted typically through one or more holes in the metal contacts of the laser diode device. This is particularly advantageous where the light emitted from the laser is to be coupled to a single mode optical fiber. The shape and size of the hole through which light is coupled to the fiber can be made to conform to the shape and size of the fiber so that less light will be diffracted. In the case of an edge laser, however, light is emitted typically through an elongated area (e.g., one micron by 0.1 micron). When light emitted from such an elongated area is coupled to a single-mode fiber, much of the light emitted will be diffracted and lost. The quality of radiation transmitted by a surface emitting laser diode device is also better than light emitted by an edge laser diode device. Furthermore, surface emitting laser devices are easier to manufacture than edge laser devices since surface emitting laser devices can be tested upon fabrication of the semiconductor wafer; the wafer need not be cut into individual diode devices before the laser diode devices thereon can be tested. In the case of edge lasers, however, no testing of the laser diode devices can be performed until each individual laser diode device has been cleaved from the wafer.
One type of surface emitting laser is described by Tai, et al., in an article, "Room Temperature Continuous Wave Vertical Cavity Surface Emitting GaAs Injection Lasers," Optical Fiber Communication Conference, Paper TUC3, 1990. In the surface emitting semiconductor laser device disclosed by Tai, an active light emitting layer is sandwiched between two groups of mirror layers for reflecting light back into the active layer to induce more light emission. Part of the light generated in the active layer is transmitted through one group of mirror layers and escapes through a hole in the substrate, and a hole in a metal contact attached to the substrate.
Light emission from the active layer is generated by passage of current there through between the two metal contacts in FIG. 1 of Tai, et al. When current is passed through the active layer, the recombination of charge carriers in the active layer causes light to be generated. To improve lasing efficiency, or in some circumstances to cause lasing action at all, it is necessary to confine the current flow between the two metal contacts to a small area of the active layer. Tai, et al. proposed the use of an insulating ring region in one of the two groups of mirror layers in order to insulate one of the metal contacts from the active layer except through a small region of the one group of mirror layers, where the region is surrounded by the ring region and is aligned with a hole in the substrate and a hole in the metal contact on the other side of the laser device. This causes current flow to be confined to a small region of the active layer aligned with a small area of the mirror layers surrounded by the insulating ring. Tai et al.'s structure does not, however, have any structure in the active layer to restrict current flow.
From the above, it is evident that Tai, et al.'s device is not entirely satisfactory. Therefore, it is desirable to provide an improved surface emitting semiconductor device in which the above-described disadvantages are not present.